1. Field of the Invention
The invention relates to self-drilling screws for fastening a softer material such as wood to a harder material such as metal. Such screws are of the type having projections, or wings, thereon to bore through the first softer material and prevent binding of the threads. In particular, the invention provides a self-drilling screw having wings of a novel configuration, which screw has faster drill times through wood than known prior art screws.
2. Description of the Prior Art
Self-drilling screws having points produced in a cold forging or pinch pointing operation are known. For example, U.S. Pat. Nos. 3,395,603; 4,295,768; 4,725,175; and 4,787,792 describe screws produced by such an operation.
Self-drilling screws for drilling and fastening a first softer work member, such as wood, to a second harder work member, such as metal, are also known. Such screws typically feature frangible wings which break off after reaming out the wood portion. For example, U.S. Pat. Nos. 3,699,841; 3,982,464; 4,347,027; 4,645,396; 4,655,661; 4,722,107; U.K. Patent 1,093,709; and DE 2,613,720 describe such self-drilling screws having fractureable wings. Self-drilling screws featuring non-breakable wings are also known. For example, U.S. Pat. No. 3,478,639 and 4,028,987 disclose such screws having non-breakable wings.
U.S. Pat. No. 3,478,639 describes an insulation stand-off screw having a dwell area between two threaded sections and protrusions, wing-like in appearance, located in the dwell area and formed in a secondary pinch point operation. The protrusions, or lugs, are basically rectangular in profile and cross-section. The working surface of the protrusion which acts as an abatement, or positive stop, is perpendicular to the axis of the threaded fastener. The protrusions act as a positive stop and do not fracture upon contact with the bottom workpiece, thereby limiting fastener penetration to avoid compression or distortion of the top insulation workpiece. The protrusions do not enhance the ability of the fastener to cut or bore either workpiece.
U.S. Pat. Nos. 3,699,841 and 3,982,464 describe self-drilling points whose typical application is to fasten two unlike materials, such as wood to steel. These points are formed in two separate operations; one to form the basic drill point, and one to form the wings via a separate forging or pinch pointing operation. Point orientation is critical in such a separate operation, and requires additional fixturing, tooling, and set-up time to position and form the wings correctly. The wings described in U.S. Pat. No. 3,699,841 are also positioned with a negative (forward) rake angle to the axis of the fastener's rotation. Such a negative rake angle will tend to inhibit the chips from escaping by acting as a dam or a blockage. In contrast, the present invention provides a drill point and wings completely formed in one forging operation with the wings having an improved configuration so as to enhance cutting action and chip escape.
U.S. Pat. No. 4,028,987 describes a self-drilling fastener having an odd shaped drill point which includes wings. The wings depicted in this patent do not fracture upon contact with the workpiece. Rather, these wings are an integral part of the drill point and must remain intact for the drill point to perform correctly. In order to effectively fasten two unlike materials, such as wood to steel, it is essential that the wings fracture on contact. Therefore, the point and wing design described in this patent does not serve to fasten two unlike materials.
U.S. Pat. Nos. 4,347,027 and U.K. 1,093,709 describe self-drilling points typically used to fasten two unlike materials, such as wood to steel. Once again, those points are formed in two separate operations; one to form the basic drill point, and one to form the wings via forging or pinch pointing. The wings of this patent are described in the primary embodiment as two diametrically opposed semi-circular slots and ears of approximate circular shape and which are formed by circular dies. A second embodiment of these wings has two diametrically opposed ears formed by dies where said ears extending from the shank have rear edges substantially normal to the line of the shank, front edges extending at a shallow angle to the line of the shank and an outer side curved for the major part. This form of ear is designed to countersink a workpiece to accommodate the head of the screw before they break away. A third embodiment is identical to the second embodiment except that a notch is provided in the front edge of each ear. In this way, each ear is divided into a front portion which countersinks and recesses the hole and a second portion of enlarged radial dimensions. Inherent in the design of these points is the necessity of increased screw length. This increases not only raw material cost, but also manufacturing costs, due to tooling required and difficulty of handling and feeding in subsequent operations, such as thread rolling. In contrast, the present invention provides a point which is completely formed in one forging operation and does not require an increase in screw length.
U.S. Pat. No. 4,645,396 describes a self-drilling point whose typical application is to fasten two unlike materials, such as wood to steel. Again, the point is formed in two separate operations, one of which is forging (pinch pointing) to form the wings. The continuous cutting edge of this design establishes the criticality of heat treatment in conjunction with wing thickness. With this design, if the heat treatment is improperly completed, the wing may not fracture immediately or cleanly upon initial contact with the bottom, or metal, workpiece, and the continuous cutting edge of this design will naturally drill into, and possibly through the metal workpiece, causing an unwanted decrease in thread engagement and holding strength. In contrast, the present invention provides a drill point which completely forms the correct hole diameter before the wings engage the bottom, or metal, workpiece, thereby creating a torque differential to easily fracture the wings and maintain proper hole diameter for optimum thread engagement and holding performance.
U.S. Pat. No. 4,655,661 describes a self-drilling point whose typical application is to fasten two unlike materials, such as fiber plate to a metal substructure. This point appears to be formed in one forging (pinch pointing) operation. The dies appear to be somewhat intricate to manufacture. This patent describes its point as a conical drill point designed with an apex angle of approximately 30.degree., with two cutting edges mounted at a distance from the point of the screw on opposing generating lines, said cutting edges being in the form of small, flat tabs with a curved, preferably approximately semi-circular outer edge. The two cutting edges are located at the same axial height on the drill point and have the same radial dimensions. The disadvantage of this point is that its design is suitable, at best, to drill or pierce relatively thin (0.040 inch) metal sub-structures.
U.S. Pat. No. 4,722,107 describes a wood-to-metal drill screw having vertical wings 30 made in the same forging, or pinch pointing, operation in which the drill point is produced. A pair of dies for shaping the point and the wing are also described. The wings are formed in rectangular cavities. As is known in the art, use of such cavities results in misformed, especially rounded or underfilled, wings. The rounding, or underfilling, results in an undesirable negative relief angle along the cutting edge. In addition, the vertical configuration does not provide a positive angle of attack. In contrast, the present invention provides a drill point and wings wherein the wings have an improved configuration including positive cutting edge relief angle and a positive angle of attack so as to enhance the cutting action and chip escape.